The present invention relates to a method for protecting a welded pipe assembly from metal dusting and a welded pipe assembly suitable for performing the method. The present invention is useful for conveying gases comprising carbon monoxide at temperatures greater than 425° C. while preventing metal dusting of the welded piping assembly.
Corrosion by metal dusting is a significant problem for industrial processes in which gases comprising carbon monoxide are conveyed at elevated temperature. Under these conditions, the gas reacts with the metal pipe. The reaction may cause rapid pitting or general wastage of the pipe alloy. Carbon diffuses into the metal often with preferential diffusion paths along the grain boundaries. The carbon can cause the grains to separate from the bulk as a metal dust rich in carbon. This corrosion mechanism is also known as catastrophic carburization.
Many industrial processes use or produce gases containing carbon monoxide. Production of these gases is typically done at elevated temperature. Examples include hydrogen or synthesis gas production by steam reforming, autothermal reforming, partial oxidation and gasification. Downstream of these production units the pipes may be subject to corrosion by metal dusting. Synthesis gas, also called syngas, is a mixture of hydrogen and carbon monoxide.
Various solutions have been proposed in order to reduce metal dusting, including the use of surface coatings which increase the corrosion resistance of the pipes. For example the interior surfaces of the pipes may be aluminized in order to limit metal dusting.
Effective aluminization is typically done by chemical vapor deposition at high temperature which results in a diffusion layer rich in aluminum on the surface of the material being coated. Fabricated components can be aluminized in this way, but there are some issues. One issue is that the size of component to be aluminized is limited by size of the furnace and the retort in which the component is to be placed. Larger furnaces and retorts are theoretically possible, but controlling the temperature uniformity necessary to achieve an effective coating becomes increasingly difficult and ultimately results in a practical limitation. Another issue is that components distort due to high temperature exposure which may limit the size and complexity of prefabricated components. So, there is a need to make final assembly of equipment in the field.
Assembling the equipment by welding piping connections presents problems when the piping and/or equipment has been coated to protect against metal dusting. This is because the welding destroys the protection of the welded region and adversely affects the protection of the adjacent region.
Industry desires methods for preventing metal dusting of welded pipe assemblies.
Industry desires methods for preventing metal dusting of welded pipe assemblies that can be applied in the field.
Industry desires welded pipe assemblies protected from metal dusting.
Industry desires welded pipe assemblies protected from metal dusting that can be constructed in the field.